The present invention relates to processes for softening cellulose pulp using chemical softening agents, including debonders such as compositions containing lower alkyl acid esters or cyclic esters of polyhydroxy compounds and polyhydroxy-functional plasticizers, products thereof, and the like.
Highly engineered absorbent articles such as premium baby diapers, adult incontinence devices, and feminine napkins are typically made with a cellulose fiber fluff-based absorbent core positioned below a liquid pervious top sheet and a low density acquisition or surge layer. The acquisition layer allows the temporary storage and unobstructed passage of fluid to the absorbent core while acting as a barrier to the retransfer of liquid back to the skin of the user. A liquid impervious backing sheet, usually of plastic material, is also provided to contain the absorbed fluid and prevent it from passing through the absorbent core and soiling the undergarments of the wearer of the absorbent article. The acquisition layer typically includes chemically stiffened cellulose fluff or bonded synthetic fibers, wherein the fibers are bonded with thermoplastic binder fibers or powder, or via the application of a latex binder.
The absorbent core of these absorbent articles is usually constructed of defiberized wood pulp with or without superabsorbent polymer granules. The absorbent core is typically formed on a pad forming unit of a converting machine on a carrier tissue to facilitate processing. Some absorbent core forming units are equipped with a layering capability in which a second discrete fluff layer may be laid over a primary fluff-based absorbent layer to form a multi-layer absorbent structure. In these absorbent structures, the primary layer may include superabsorbent polymer granules. Examples of conventionally produced absorbent structures include those described in U.S. Pat. Nos. 5,009,650; 5,378,528; 5,128,082; 5,607,414; 5,147,343; 5,149,335; 5,522,810; 5,041,104; 5,176,668; 5,389,181; and 4,596,567.
The manufacture of disposable absorbent hygienic products, particularly diapers and adult incontinence products, is usually performed on a continuous production line in which the cellulose fluff absorptive material is supplied as a roll of comminution pulp. The pulp is manufactured by conventional wet-laid techniques, wherein the pulp sheet is unrolled and fed into a hammer mill or similar mechanical apparatus to separate the cellulose fibers in the sheet into cellulose fluff. A drying stage may or may not be needed ahead of the hammer mill, depending on the needs of the skilled artisan employing the process of the invention. The fluff is then conveyed to the forming area where it is air-laid in the amount and shape desired in the final product.
To soften the sheeted product for efficient comminution, cationic surfactants have been used traditionally as debonders to disrupt interfiber associations, thereby producing a softer and weaker sheeted product. Examples of debonders are disclosed in U.S. Pat. No. 4,432,833; U.S. Pat. No. 4,425,186; and U.S. Pat. No. 5,776,308. A common drawback to the conventional cationic debonders is a loss of wettability or absorbency of the comminution pulp due to the relatively long alkyl chain of the cationic surfactant. Blocking hydrogen-bonding sites softens the pulp sheet so it is more easily comminuted into individual fibers. Debonders also encourage the formation of lofty, low density airlaid structures which resist permanent densification since the hydrogen bonds between fibers are blocked by the debonder molecules. Thus, it would be advantageous to produce a comminuted pulp product which is easily densified for use in a final air-laid or woven product, without loss of wettability or absorbency.
Plasticizers for cellulose, which can be added to a pulp slurry prior to forming wetlaid sheets, can also be used to soften pulp, although they act by a different mechanism than debonding agents. Plasticizing agents act within the fiber, at the cellulose molecule, to make flexible or soften amorphous regions. The resulting fibers are characterized as limp. Since the plasticized fibers lack stiffness, the comminuted pulp is easier to densify compared to fibers not treated with plasticizer.
Plasticizers include polyhydric alcohols such as glycerol; low molecular weight polyglycols such as polyethylene glycols and polypropylene glycols; and other polyhydroxy compounds. These and other plasticizers are described and exemplified in U.S. Pat. No. 4,098,996; U.S. Pat. No. 5,547,541; and U.S. Pat. No. 4,731,269. Ammonia, urea, and alkylamines are also known to plasticize wood products, which mainly contain cellulose (A. J. Stamm, Forest Products Journal 5(6):413, 1955.
Plasticizing provides for easier densification of airlaid nonwovens made from treated pulp after comminution. A method of softening a cellulose composition that effectively debonds pulp fibers for efficient comminution, thereby making it easier to refiberize and subsequently densify without decreasing wettability would be highly beneficial, but is lacking in the art.
The present invention provides a novel process, and products thereof, for softening cellulose pulp by using a new combination of chemical softening agents, which includes both a debonder and a plasticizer. This softening treatment converts ordinary fluff pulp sheets into softened sheets of limp fibers having little affinity for each other (i.e., plasticized and debonded pulp). Thus, the resulting pulp is both easier to fluff (refiberize) and subsequently densify into airlaid pads formed from the individualized fibers after comminution. In addition, the absorbency and wettability of the cellulose fibers is not compromised by the softening process of the invention.
Practice of the invention using both a debonder and a plasticizer considerably lowers the energy requirement for converting pulp fiber sheets into absorbent products. Thus, the process of the invention reduces the cost of manufacturing products produced from refiberized comminuted pulp as a result of lower energy demand, higher throughput, and decreased wear on equipment. These are surprising and unexpected benefits of the present invention which could not previously be obtained from using either a conventional debonder or plasticizer alone to soften pulp.
In one embodiment, the invention provides a process for softening cellulose pulp comprising the step of contacting the pulp with an aqueous solution containing a debonder and a plasticizer in combination.
In another embodiment the invention provides a novel debonding agent, propylene carbonate, for use as a debonder alone, or together with a plasticizer in accordance with the process of the invention for softening cellulose pulp.
The invention further provides a softened pulp produced according to the processes of the invention for use in an absorbent product.